Corrosion-Resistant Austenitic Steel

ABSTRACT

A corrosion-resistant austenitic steel is claimed which, in each case relative to 100 mass percent, contains 20 to 32% manganese, 10 to 15% chromium, a total of 0.5 to 1.3% carbon and nitrogen, wherein the ratio of carbon to nitrogen is 0.5 to 1.5, the remainder being iron and melt-related impurities. The claimed steel can be produced and processed at normal pressure and has TWIP properties. It is in particular suited for producing structural components in constructs, such as in the automotive industry.

The present invention concerns a corrosion-resistant austenitic steel, a method for its production, and the use of said steel.

The strength of austenitic steels is increased in particular by the interstitially dissolved atoms of the elements carbon and nitrogen. In order to dissolve the volatile element nitrogen in the melt, in general chromium and manganese are added to the alloy. While chromium enhances solely the ferrite formation, with manganese, by so-called solution annealing, an austenitic structure can be obtained that is stabilized by quenching to room temperature.

An austenitic steel grade is the so-called TWIP steel (twinning induced plasticity, in German “Zwillingsbildung induzierte Platistizät”) that exhibits an intensive twinning when undergoing plastic deformation. This process generally occurs already at minimal load and hardens the steel wherein the elongation at fracture is above 60%. As a result of these properties, the steel is suitable excellently for producing sheet metal in the automotive industry, in particular for accident-relevant parts of the car body. The TWIP steel has in general a carbon contents of approximately 0.02 to 0.5% by weight; as alloying elements manganese in quantities of 20 to 30% by weight as well as, in certain TWIP steels, aluminum and silicon, each with up to 3% by weight, are used.

EP 0 889 144 discloses a so-called TWIP steel, a lightweight construction steel, that has a tensile strength of up to 1,100 MPa and contains 1 to 6% by weight Si, 1 to 8% by weight of Al, wherein the total contents of Al and Si is not greater than 12% by weight, as well as 10 to 30% by weight of Mn. The disclosed steels are distinguished by higher yield stress of 400 MPa as well as uniform strain values up to 70% and elongation at fracture up to 90%. A disadvantage of the steel disclosed in this document is the minimal corrosion resistance.

In DE 101 a high-strength, stainless austenitic steel is characterized in that it is melted under normal atmospheric pressure of approximately 1 bar and, in addition to iron, contains 12 to 15% by weight of chromium, 17 to 21% by weight of manganese, <0.7% by weight of silicon, 0.4 to 0.7% by weight of carbon and nitrogen in sum, and <1.0% by weight of further production-related elements in sum, wherein the ratio of carbon content and nitrogen content is between 0.6 and 1.0. The disclosed steel exhibits no TWIP effect and may form martensite at strong deformation, which is expressed inter alia in a minimal nominal strain.

WO 2006/025412 discloses a corrosion-resistant TWIP steel that contains Fe, Al, Si, Mn, Cr, and Ni as main elements. The obtained steel shows uniform strain values above 50% and a tensile strength between 600 and 800 MPa. The mechanical properties are comparable to those of the steel disclosed in EP 0 889 144 on the basis of Fe, Al, Si, and Mn but the addition of nickel increases the production costs and the lack of interstitial atoms leads to a minimal strength. A further austenitic steel that contains C and N as alloying elements is disclosed in WO 2006/027091 wherein the steel described therein contains, in addition to the alloying metals chromium and manganese, each in quantities of 16 to 21% by weight, also 0.5 to 2.0% by weight of molybdenum as well as a total of 0.8 to 1.1% by weight of carbon and nitrogen with a carbon/nitrogen ratio of 0.5 to 1.1. The disclosed steel exhibits mechanical strength, ductility, wear and corrosion resistance and no ferromagnetism. A disadvantage is however that, upon solidification during the production of these alloys, a primary ferrite formation occurs that may cause escape of nitrogen during melting and/or welding.

The present invention was based on the object to provide a corrosion-resistant weldable austenitic steel that has a high tensile elastic limit and also a high tensile strength as well as an elongation at fracture of above 90% and that is, at the same time, corrosion-resistant.

Object of the present invention is a corrosion-resistant austenitic steel containing, in addition to iron, based on 100 percent by weight,

20 to 32% of manganese

10% to 15% of chromium, a total of 0.5 to 1.3% of carbon and nitrogen, wherein the ratio of carbon to nitrogen is 0.5 to 1.5,

as well as melt-related impurities.

The austenitic steel according to the invention exhibits the TWIP properties (TWIP=twinning induced plasticity) as well as good corrosion resistance. A significant property of this TWIP steel is to obtain a plasticity by formation of twinned grain boundaries with excellent corrosion resistance; this means a steel that upon deformation forms numerous twinned grain boundaries in its microstructure, thereby hardens greatly and uniformly, exhibits high nominal strain values in a tensile test, as well as remains completely austenitic without formation of martensite.

The steel according to the invention exhibits a stabilized austenitic structure that is formed by combination of the main alloying elements Fe, Mn, and Cr as well as the interstitial elements C and N. The steel according to the invention shows in tensile test an elongation at fracture of above 90%, a tensile elastic limit of above 400 MPa, and a tensile strength of above 900 MPa. Because of the combination of high elongation at fracture and tensile elastic limit, the steel according to the present invention is extremely ductile. Moreover, the alloys according to the present invention exhibit no α martensite or ε martensite formation detectable by x-ray diffraction after a targeted deformation.

It was found that the alloy according to the invention with the above described proportions of Cr, Mn, C, and N enables a primary austenitic solidification so that a melt is obtained from which nitrogen will not escape during solidification and/or welding. The alloy can thus be produced and also processed under normal pressure. The alloy according to the invention exhibits a stable austenitic structure that prevents formation of ferrite. The alloying metal Cr and the contained N effect a higher corrosion resistance in comparison to TWIP steels of the prior art.

The individual quantity ratios of the alloying elements Cr and Mn as well as the additives N and C are adjusted in such a ratio that the quantity of Cr not only improves the solubility of N in the melt but also advantageously effects the corrosion resistance of the alloy without ferrite being formed primarily upon solidification of the melt. The formation of ferrite is disadvantageous because it provides a reduced solubility for nitrogen and thus causes pore formation. Moreover, in the alloy according to the invention the formation of precipitations, for example, carbides and nitrides, was shifted to lower temperatures. This enables a slower cooling from the austenitization temperature as well as an unproblematic manufacture of larger component cross-sections.

Also, the weldability of the alloy according to the invention is affected positively by avoiding nitrogen gas escape during solidification after fusion welding as well as by avoiding the formation of precipitations during the subsequent cooling of the solid material of the weld seam and of the heat-affected zone to room temperature. This is important primarily for technological reasons because the material cools relatively slowly after welding and formation of precipitations at the weld seam and in the heat-affected zone is undesirable.

The quantity of Mn improves the ductility (plasticity, shape-changing capability). The further components C and N improve the mechanical properties and the corrosion resistance without nitrides and carbides being formed. The inventive ratio of C and N enables a completely austenitic solidification without gases escaping during melting or carbides or nitrides being formed during accelerated cooling. The solubility for the desired quantity of nitrogen in the melt is preferably realized at 1,500° C. and a pressure of 1 bar.

In a preferred embodiment the alloying metals are present in a quantity of 22.0 to 30.0% by weight of Mn and in a quantity of 11.0 to 13% by weight of chromium, in particular 12.0 to 13% by weight. A total content of carbon and nitrogen between 0.5 to 0.8% by weight at a ratio of carbon to nitrogen of 0.5 to 0.8 has been found to be particularly beneficial. The alloys of this embodiment exhibit advantageous material properties so that they are suitable for use in lightweight construction.

In a further embodiment, the alloy according to the invention contains secondary alloying metals with which the mechanical properties can be further adjusted. The secondary alloying elements are preferably selected from Mo, Si, Nb, Hf, V, Zr, Ti, and Nd. Of these alloying metals, Mo is preferably contained in a quantity of 1.0 to 2.0% by weight, Si in a quantity of 0.1 to 2% by weight. The metals Nb, Hf, V, Zr, Ti, and Nd may be contained in smaller quantities and are also referred to as microalloying elements. Of the microalloying elements, Nb can be present in a quantity of 0.02 to 0.1% by weight, the metals Hf, V, Zr, Ti, and Nd each, independent of each other, in quantities of 0 to 0.5% by weight.

A further object of the present invention is a method for producing a corrosion-resistant austenitic steel with TWIP properties in which the individual alloying metals are melted under normal pressure and diffusion annealing is carried out in a temperature range between 1,000 and 1,250° C. for a duration of 1 to 72 hours with subsequent quenching and hot/cold deformation.

The melting process can be performed at a pressure of 800-1,000 mbar in pure nitrogen or in a furnace at environmental pressure which corresponds to a partial pressure of nitrogen of approximately 800 mbar.

A further object of the present invention concerns the use of the austenitic steel according to the invention for producing structural components in structures, in particular in the automotive industry.

EXAMPLES

In the following Table 1 examples of alloys according to the invention are represented.

TABLE 1 contents in % by weight Fe Mn Cr C N Nb Mo C + N C/N grade 1 bal. 30.0 12.0 0.3 0.4 0 0 0.56 0.75 30Mn12CrCN 2 bal. 25.0 12.0 0.3 0.4 0 0 0.7 0.75 25Mn12CrCN 3 bal. 20.0 12.0 0.24 0.32 0 0 0.7 0.75 20Mn12CrCN 4 bal. 25.0 12.0 0.3 0.4 0.05 0 0.7 0.75 — 5 bal. 25.0 12.0 0.3 0.4 0.05 0.5 0.7 0.75 — 6 bal. 25.0 12.0 0.3 0.4 0.05 1.0 0.7 0.75 — 7 bal. 25.0 12.0 0.3 0.4 0.05 1.5 0.7 0.75 —

The mechanical properties are listed in Table 2.

0.2% yield tensile strength in strength in elongation at Vickers hardness example MPa MPa fracture in % in HV10 30Mn12CrCN 449.76 906.92 108.5 234.2 25Mn12CrCN 445.38 889.96 93.8 230.3 20Mn12CrCN 434.00 825.08 93.1 200.6

In the subsequent diagrams the stress-strain curves under load at room temperature (diagram 1), impact strength (diagram 2), and a computed phase diagram in which the primary austenite formation can be seen (diagram 3) are illustrated graphically. 

1-15. (canceled)
 16. A corrosion-resistant austenitic steel comprising, in addition to iron, in percent by weight based on 100 percent by weight of the steel: 20 to 32% of manganese, 10 to 15% of chromium, a total of 0.5 to 1.3% carbon and nitrogen, wherein the ratio of carbon to nitrogen is 0.5 to 1.5, and melt-related impurities.
 17. The corrosion-resistant austenitic steel according to claim 16, further comprising alloying components selected from the group consisting of Mo, Si, Nb, Hf, V, Zr, Ti, Nd, and Co.
 18. The corrosion-resistant austenitic steel according to claim 17, wherein Mo is contained in a quantity of 1.0 percent by weight to 2.0 percent by weight.
 19. The corrosion-resistant austenitic steel according to claim 17, wherein Si is contained in a quantity of 0.1 percent by weight to 2.0 percent by weight.
 20. The corrosion-resistant austenitic steel according to claim 17, wherein Nb is contained in a quantity of 0.02 percent by weight to 0.1 percent by weight.
 21. The corrosion-resistant austenitic steel according to claim 17, wherein Hf, V, Zr, Ti, and Nd each are contained in a quantity of up to 0.5 percent by weight.
 22. The corrosion-resistant austenitic steel according to claim 16, wherein manganese is contained in a quantity of 22 percent by weight to 30 percent by weight.
 23. The corrosion-resistant austenitic steel according to claim 16, wherein chromium is contained in a quantity of 11.0 percent by weight to 13.0 percent by weight.
 24. The corrosion-resistant austenitic steel according to claim 16, wherein carbon and nitrogen are contained in total in a quantity of 0.5 percent by weight to 0.8 percent by weight and wherein the ratio of carbon to nitrogen is between 0.5 and 0.8.
 25. The corrosion-resistant austenitic steel according to claim 16 having TWIP properties.
 26. The corrosion-resistant austenitic steel according to claim 16 having a tensile strength of >900 MPa.
 27. The corrosion-resistant austenitic steel according to claim 16 having a tensile elastic limit of >400 MPa and an elongation at fracture of >90%.
 28. A method for producing a corrosion-resistant austenitic steel, the method comprising the steps of: melting alloying metals at normal pressure, annealing the alloying metals in a temperature range between 1,000° C. and 1,250° C. for a duration of 1 to 72 hours, and quenching the alloying metals.
 29. The method according to claim 13, further comprising, subsequent to quenching, a deformation step employing hot deforming; cold deforming; or hot deforming and cold deforming.
 30. A method for producing structural components, the method comprising the step of providing a corrosion-resistant austenitic steel according to claim
 16. 